U.S. patent number 6,202,431 [Application Number 09/232,558] was granted by the patent office on 2001-03-20 for adaptive hot gas bypass control for centrifugal chillers.
This patent grant is currently assigned to York International Corporation. Invention is credited to Gregory K. Beaverson, Dennis L. Deltz, Harold B. Ginder.
United States Patent |
6,202,431 |
Beaverson , et al. |
March 20, 2001 |
**Please see images for:
( Certificate of Correction ) ** |
Adaptive hot gas bypass control for centrifugal chillers
Abstract
An adaptive control apparatus and a method for automatically
controlling a refrigeration system as a function of cooling load
and head. A control panel controls the operation of a hot gas
bypass valve so as to avoid surging of the compressor in response
to cooling load and head. The control apparatus and method also
allow for automatic self calibration.
Inventors: |
Beaverson; Gregory K. (York,
PA), Ginder; Harold B. (York, PA), Deltz; Dennis L.
(Windsor, PA) |
Assignee: |
York International Corporation
(York, PA)
|
Family
ID: |
22873624 |
Appl.
No.: |
09/232,558 |
Filed: |
January 15, 1999 |
Current U.S.
Class: |
62/196.3; 62/201;
62/209 |
Current CPC
Class: |
F25B
49/02 (20130101); F25B 1/053 (20130101); F04D
27/0207 (20130101); F25B 2600/0261 (20130101) |
Current International
Class: |
F04D
27/02 (20060101); F25B 1/04 (20060101); F25B
1/053 (20060101); F25B 49/02 (20060101); F25B
41/04 (20060101); F25B 041/00 () |
Field of
Search: |
;62/126,129,196.1,196.3,203,204,217,228.1,228.3,228.4,228.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1-281353 |
|
Nov 1989 |
|
JP |
|
4-260755 |
|
Sep 1992 |
|
JP |
|
4-297761 |
|
Oct 1992 |
|
JP |
|
5-52433 |
|
Mar 1993 |
|
JP |
|
6-185786 |
|
Jul 1994 |
|
JP |
|
Other References
PCT International Search Report, International Application No.
PCT/US00/00729, Apr. 21, 2000, 5 pages..
|
Primary Examiner: Tanner; Harry B.
Attorney, Agent or Firm: Finnegan, Henderson, Farabow,
Garrett & Dunner, L.L.P.
Claims
What is claimed is:
1. A method for automatically calibrating a surge control of a
refrigeration system including a centrifugal compressor, a
condenser, pre-rotational vanes, a load, and an evaporator through
which a chilled liquid refrigerant is circulated, said method
comprising:
sensing a presence of a surge condition;
sensing a head parameter representative of the head of the
compressor;
sensing a load parameter representative of the load; and
storing the head parameter and the load parameter when the surge
condition is sensed as calibration data to be used by the control
of the refrigeration system;
sensing a present head parameter representative of the present head
of the compressor;
sensing a present load parameter representative of the present
load; and
controlling the operation of a hot gas bypass valve so as to avoid
surging in the compressor in response to the present head
parameter, the present load parameter, and the stored control
calibration data.
2. The method of claim 1, wherein sensing the head parameter
includes
sensing a pressure representative of the pressure of the liquid
refrigerant in the condenser;
sensing a pressure representative of the pressure of the liquid
refrigerant in the evaporator;
calculating a differential pressure equal to the difference between
the condenser pressure and the evaporator pressure; and
calculating a pressure ratio equal to the ratio between the
calculated differential pressure and the evaporator pressure.
3. The method of claim 1, wherein sensing the load parameter
includes sensing a position representative of the position of the
pre-rotational vanes.
4. The method of claim 1, wherein sensing the head parameter
includes
sensing a pressure representative of the pressure of the liquid
refrigerant in the condenser;
sensing a pressure representative of the pressure of the liquid
refrigerant in the evaporator;
calculating a differential pressure equal to the difference between
the condenser pressure and the evaporator pressure; and
calculating a pressure ratio equal to the ratio between the
calculated differential pressure and the evaporator pressure;
and
wherein sensing the load parameter includes sensing a position
representative of the position of the pre-rotational vanes.
5. The method of 4, wherein storing the head parameter includes
storing the pressure ratio, minus a small margin, as a stored
control pressure ratio when the surge condition is sensed; and
storing the corresponding vane position as a stored control vane
position when the surge condition is sensed.
6. The method of claim 1, wherein sensing the present head
parameter includes
sensing a present pressure representative of the present pressure
of the liquid refrigerant in the condenser;
sensing a present pressure representative of the present pressure
of the liquid refrigerant in the evaporator;
calculating a present differential pressure equal to the difference
between the present condenser pressure and the present evaporator
pressure; and
calculating a pressure ratio equal to the ratio between the present
calculated differential pressure and the present evaporator
pressure.
7. The method of claim 1, wherein sensing the present load
parameter includes
sensing a present position representative of the present position
of the pre-rotational vanes.
8. The method of claim 1, wherein sensing the present head
parameter includes
sensing a present pressure representative of the present pressure
of the liquid refrigerant in the condenser;
sensing a present pressure representative of the present pressure
of the liquid refrigerant in the evaporator;
calculating a present differential pressure equal to the difference
between the present condenser pressure and the present evaporator
pressure;
calculating a present pressure ratio equal to the ratio between the
present calculated differential pressure and the present evaporator
pressure; and
sensing a present position representative of the present position
of the pre-rotational vanes.
9. The method of claim 8, wherein the stored control calibration
data includes a stored control pressure ratio and a stored control
vane position, said method including
opening the hot gas bypass valve, if the current pressure ratio is
greater than the stored control pressure ratio corresponding to the
stored control vane position equal to the current vane position, by
an amount proportional to a difference between the current pressure
ratio and the stored control pressure ratio.
10. The method of claim 8 wherein the stored calibration data
includes a stored control pressure ratio and a stored control vane
position, said method including
closing completely the hot gas bypass valve, if the current
pressure ratio is less than or equal to the stored control pressure
ratio corresponding to the stored control vane position equal to
the current vane position.
11. A method for controlling a hot gas bypass valve in a
refrigeration system including a centrifugal compressor, a
condenser, pre-rotational vanes, and an evaporator through which a
chilled liquid refrigerant is circulated, said method
comprising:
sensing a present pressure representative of the present pressure
of the liquid refrigerant in the condenser;
sensing a present pressure representative of the present pressure
of the liquid refrigerant in the evaporator;
sensing a present vane position representative of the present
position of the pre-rotational vanes; and
controlling the operation of the hot gas bypass valve so as to
avoid surging in the compressor in response to the present
condenser pressure, the present evaporator pressure, and the
present vane position to stored calibration data.
12. The method of claim 11, wherein controlling the operation
includes
calculating a present differential pressure equal to the difference
between the present condenser pressure and the present evaporator
pressure; and
calculating a present pressure ratio equal to the ratio between the
present calculated differential pressure and the present evaporator
pressure.
13. The method of claim 11, wherein stored calibration data
includes stored control pressure ratios and stored control vane
position, said method including
opening the hot gas bypass valve, if the present pressure ratio is
greater than the stored control pressure ratio corresponding to the
stored control vane position equal to the present vane position, by
an amount proportional to a difference between the present pressure
ratio and the stored control pressure ratio.
14. The method of claim 11, wherein stored calibration data
includes stored control pressure ratios corresponding stored
control vane positions, said method including
closing completely the hot gas bypass valve, if the present
pressure ratio is less than or equal to the stored control pressure
ratio corresponding to the stored control vane position equal to
the present vane position.
15. An apparatus for automatically calibrating a surge control of a
refrigeration system including a centrifugal compressor, a
condenser, pre-rotational vanes, a load, and an evaporator through
which a chilled liquid refrigerant is circulated, said method
comprising:
means for sensing a presence of a surge condition;
means for sensing a head parameter representative of the head of
the compressor;
means for sensing a load parameter representative of the load;
means for storing the head parameter and the load parameter when
the surge condition is sensed as calibration data to be used by the
control of the refrigeration system;
means for sensing a present head parameter representative of the
present head of the compressor;
means for sensing a present load parameter representative of the
present load; and
means for controlling the operation of a hot gas bypass valve so as
to avoid surging in the compressor in response to the present head
parameter, the present load parameter, and the stored control
calibration data.
16. The apparatus of claim 15, wherein means for sensing the head
parameter includes
means for sensing a pressure representative of the pressure of the
liquid refrigerant in the condenser;
means for sensing a pressure representative of the pressure of the
liquid refrigerant in the evaporator;
means for calculating a differential pressure equal to the
difference between the condenser pressure and the evaporator
pressure; and
means for calculating a pressure ratio equal to the ratio between
the calculated differential pressure and the evaporator
pressure.
17. The apparatus of claim 15, wherein means for sensing the load
parameter includes
means for sensing a position representative of the position of the
pre-rotational vanes.
18. The apparatus of claim 15, wherein means for sensing the head
parameter includes means for sensing a pressure representative of
the pressure of the liquid refrigerant in the condenser;
means for sensing a pressure representative of the pressure of the
liquid refrigerant in the evaporator;
means for calculating a differential pressure equal to the
difference between the condenser pressure and the evaporator
pressure; and
means for calculating a pressure ratio equal to the ratio between
the calculated differential pressure and the evaporator pressure;
and
wherein means for sensing the load parameter includes means for
sensing a position representative of the position of the
pre-rotational vanes.
19. The apparatus of 18, wherein means for storing the head
parameter includes
means for storing the pressure ratio, minus a small margin, as a
stored control pressure ratio when the surge condition is sensed;
and
means for storing the corresponding vane position as a stored
control vane position when the surge condition is sensed.
20. The apparatus of claim 15, wherein means for sensing the
present head parameter includes
means for sensing a present pressure representative of the present
pressure of the liquid refrigerant in the condenser;
means for sensing a present pressure representative of the present
pressure of the liquid refrigerant in the evaporator;
means for calculating a present differential pressure equal to the
difference between the present condenser pressure and the present
evaporator pressure; and
means for calculating a present pressure ratio equal to the ratio
between the present calculated differential pressure and the
present evaporator pressure.
21. The apparatus of claim 15, wherein means for sensing the
present load parameter includes
means for sensing a present position representative of the present
position of the pre-rotational vanes.
22. The apparatus of claim 15, wherein means for sensing the
present head parameter includes
means for sensing a present pressure representative of the present
pressure of the liquid refrigerant in the condenser;
means for sensing a present pressure representative of the present
pressure of the liquid refrigerant in the evaporator;
means for calculating a present differential pressure equal to the
difference between the present condenser pressure and the present
evaporator pressure;
means for calculating a present pressure ratio equal to the ratio
between the present calculated differential pressure and the
present evaporator pressure; and
means for sensing a present position representative of the present
position of the pre-rotational vanes.
23. The apparatus of claim 22, wherein the stored control
calibration data includes a stored control pressure ratio and a
stored control vane position, said apparatus including
means for opening the hot gas bypass valve, if the present pressure
ratio is greater than the stored control pressure ratio
corresponding to the stored control vane position equal to the
present vane position, by an amount proportional to a difference
between the present pressure ratio and the stored control pressure
ratio.
24. The apparatus of claim 22 wherein the stored calibration data
includes a stored control pressure ratio and a stored control vane
position, said apparatus including
means for closing completely the hot gas bypass valve, if the
present pressure ratio is less than or equal to the stored control
pressure ratio corresponding to the stored control vane position
equal to the present vane position.
25. An apparatus for controlling a hot gas bypass valve in a
refrigeration system including a centrifugal compressor, a
condenser, pre-rotational vanes, and an evaporator through which a
chilled liquid refrigerant is circulated, said apparatus
comprising:
means for sensing a present pressure representative of the present
pressure of the liquid refrigerant in the condenser;
means for sensing a present pressure representative of the present
pressure of the liquid refrigerant in the evaporator;
means for sensing a present vane position representative of the
present position of the pre-rotational vanes; and
means for controlling the operation of a hot gas bypass valve so as
to avoid surging in the compressor in response to the present
condenser pressure, the present evaporator pressure, and the
present vane position to stored calibration data.
26. The apparatus of claim 25, wherein means for controlling the
operation includes
means for calculating a present differential pressure equal to the
difference between the present condenser pressure and the present
evaporator pressure; and
means for calculating a present pressure ratio equal to the ratio
between the present calculated differential pressure and the
present evaporator pressure.
27. The apparatus of claim 25, wherein stored calibration data
includes stored control pressure ratios and stored control vane
position, said apparatus including
means for opening the hot gas bypass valve, if the present pressure
ratio is greater than the stored control pressure ratio
corresponding to the stored control vane position equal to the
present vane position, by an amount proportional to a difference
between the present pressure ratio and the stored control pressure
ratio.
28. The apparatus of claim 25, wherein stored calibration data
includes stored control pressure ratios corresponding to stored
control vane positions, said apparatus including
means for closing completely the hot gas bypass valve, if the
present pressure ratio is less than or equal to the stored control
pressure ratio corresponding to the stored control vane position
equal to the present vane position.
29. A refrigeration system including a centrifugal compressor, a
condenser, pre-rotational vanes, a hot gas bypass valve, and an
evaporator through which a chilled liquid refrigerant is
circulated, said apparatus comprising:
means for sensing a present pressure representative of the present
pressure of the liquid refrigerant in the condenser;
means for sensing a present pressure representative of the present
pressure of the liquid refrigerant in the evaporator;
means for sensing a present position representative of the present
position of the pre-rotational vanes; and
means for controlling the operation of the hot gas bypass valve so
as to avoid surging in the compressor in response to a comparison
of the present condenser pressure, the present evaporator pressure,
and the present vane position, or functions thereof, to stored
calibration data.
30. The apparatus of claim 29, wherein means for controlling the
operation includes
means for calculating a present differential pressure equal to the
difference between the present condenser pressure and the present
evaporator pressure; and
means for calculating a present pressure ratio equal to the ratio
between the present calculated differential pressure and the
present evaporator pressure.
31. The apparatus of claim 29, wherein stored calibration data
includes stored control pressure ratios and stored control vane
position, said apparatus including
means for opening the hot gas bypass valve, if the present pressure
ratio is greater than the stored control pressure ratio
corresponding to the stored control vane position equal to the
present vane position, by an amount proportional to a difference
between the present pressure ratio and the stored control pressure
ratio.
32. The apparatus of claim 29, wherein stored calibration data
includes stored control pressure ratios corresponding stored
control vane positions, said apparatus including
means for closing completely the hot gas bypass valve, if the
present pressure ratio is less than or equal to the stored control
pressure ratio corresponding to the stored control vane position
equal to the present vane position.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to refrigerating systems or
chilling systems, and more particularly, to an apparatus and method
for controlling a hot gas bypass valve to eliminate or minimize
surge in centrifugal liquid chilling systems.
2. Description of the Related Art
As is generally known, surge or surging is an unstable condition
that may occur when compressors, such as centrifugal compressors,
are operated at light loads and high pressure ratios. It is a
transient phenomenon characterized by high frequency oscillations
in pressures and flow, and, in some cases, a complete flow reversal
through the compressor. Such surging, if uncontrolled, causes
excessive vibrations and may result in permanent compressor damage.
Further, surging causes excessive electrical power consumption if
the drive device is an electric motor.
It is generally known that a hot gas bypass flow helps avoid
surging of the compressor during low-load or partial load
conditions. As the cooling load decreases, the requirement for hot
gas bypass flow increases. The amount of hot gas bypass flow at a
certain load condition is dependent on a number of parameters,
including the desired head pressure of the centrifugal compressor.
Thus, it is desirable to provide a control system for the hot gas
bypass flow that provides optimum control and is responsive to the
characteristic of a given centrifugal chiller system.
An hot gas bypass valve control in the prior art is an analog
electronic circuit described in U.S. Pat. No. 4,248,055. This prior
art control provides as its output a DC voltage signal that is
proportional to the required amount of opening of the valve. This
prior art method requires calibration at two different chiller
operating points at which the compressor just begins to surge. As a
consequence of this, a good deal of time is consumed performing the
calibration and it requires the assistance of a service technician
at the chiller site. Further, variation of flow is necessary for
many applications, and therefore, repeated calibration of the
control is required. Another disadvantage of the prior art method
is that it makes the false assumption that the surge boundary is a
straight line. Instead, it is often characterized by a curve that
may deviate significantly from a straight line at various operating
conditions. As a consequence of this straight line assumption, the
hot gas bypass valve may open too much or too little. Opening the
valve too much may result in inefficient operation, and opening it
too little may result in a surge condition.
SUMMARY OF THE INVENTION
The advantages and purpose of the invention are set forth in part
in the description that follows, and in part is obvious from the
description, or may be learned by practice of the invention. The
advantages and purpose of the invention is realized and attained by
means of the elements and combinations particularly pointed out in
the claims.
To attain the advantages and in accordance with the purpose of the
invention, as embodied and broadly described herein, systems and
methods consistent with this invention automatically calibrate a
surge control of a refrigeration system including a centriftigal
compressor, a condenser, pre-rotational vanes, a load, and an
evaporator through which a chilled liquid refrigerant is
circulated. The system or method comprises a number of elements.
First, systems or methods consistent with this invention sense a
presence of a surge condition, sense a head parameter
representative of the head of the compressor, and sense a load
parameter representative of the load. Second, systems or methods
consistent with this invention store the head parameter and the
load parameter when the surge condition is sensed as calibration
data to be used by the control of the refrigeration system.
To attain the advantages and in accordance with the purpose of the
invention, as embodied and broadly described herein, systems and
methods consistent with this invention control a hot gas bypass
valve in a refrigeration system including a centrifugal compressor,
a condenser, pre-rotational vanes, and an evaporator through which
a chilled liquid refrigerant is circulated. The system or method
comprises a number of elements. First, systems or methods
consistent with this invention sense a current pressure
representative of the current pressure of the liquid refrigerant in
the condenser, sense a current pressure representative of the
current pressure of the liquid refrigerant in the evaporator, and
sense a current position representative of the current position of
the pre-rotational vanes. Second, systems or methods consistent
with this invention control the operation of a hot gas bypass valve
so as to avoid surging in the compressor in response to a
comparison of the current condenser pressure, the current
evaporator pressure, and the current vane position, or functions
thereof, to stored calibration data.
The summary and the following detailed description should not
restrict the scope of the claimed invention. Both provide examples
and explanations to enable others to practice the invention. The
accompanying drawings, which form part of the detailed description,
show one embodiment of the invention and, together with the
description, explain the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute
a part of this specification, illustrate one embodiment of the
invention and together with the description, serve to explain the
principles of the invention. In the drawings,
FIG. 1 is a diagram of a refrigeration system and control panel
consistent with this invention;
FIG. 2 is a diagram of a table that stores control pressure ratios
and corresponding prerotational rotational vane position index and
a plot of the values in the table, each consistent with this
invention;
FIGS. 3A, 3B, 3C are a flow diagram of the Adaptive Hot Gas Bypass
control process consistent with this invention;
FIGS. 4A, 4B, 4C are a flow diagram for the sub-process of
recording or storing control pressure ratios in a table as shown in
FIG. 2;
FIGS. 5A, 5B, 5C are a flow diagram for a hot gas bypass valve
control sub-process consistent with this invention; and
FIG. 6 is a flow diagram for a sub-process for determining the PRV
index shown in of FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The following description of embodiments of this invention refers
to the accompanying drawings. Where appropriate, the same reference
numbers in different drawings refer to the same or similar
elements.
FIG. 1 is a diagram of a refrigeration system 100 and control panel
consistent with this invention. Refrigeration system 100 includes a
centrifugal compressor 110 that compresses the refrigerant vapor
and delivers it to a condenser 112 via line 114. The condenser 112
includes a heat-exchanger coil 116 having an inlet 118 and an
outlet 120 connected to a cooling tower 122. The condensed liquid
refrigerant from condenser 112 flows via line 124 to an evaporator
126. The evaporator 126 includes a heat-exchanger coil 128 having a
supply line 128S and a return line 128R connected to a cooling load
130. The vapor refrigerant in the evaporator 126 returns to
compressor 110 via a suction line 132 containing pre-rotational
vanes (PRV) 133. A hot gas bypass (HGBP) valve 134 is
interconnected between lines 136 and 138 which are extended from
the outlet of the compressor 110 to the inlet of PRV 133.
A control panel 140 includes an interface module 146 for opening
and closing the HGBP valve 134. Control panel 140 includes an
analog to digital (A/D) converter 148, a microprocessor 150, a
non-volatile memory 144, and an interface module 146.
A pressure sensor 154 generates a DC voltage signal 152
proportional to condenser pressure. A pressure sensor 160 generates
a DC voltage signal 162 proportional to evaporator pressure.
Typically these signals 152, 162 are between 0.5 and 4.5V (DC). A
PRV position sensor 156 is a potentiometer that provides a DC
voltage signal 158 that is proportional to the position of the PRV.
A temperature sensor 170 on supply line 128S generates a DC voltage
signal 168 proportional to leaving chilled liquid temperature. The
four DC voltage signals 158, 152, 162, and 168 are inputs to
control panel 140 and are each converted to a digital signal by A/D
converter 148. These digital signals representing the two
pressures, the leaving chilled liquid temperature, and the PRV
position are inputs to microprocessor 150.
Microprocessor 150 performs with software all necessary
calculations and decides what the HGBP valve position should be, as
described below, as well as other functions. One of these functions
is to electronically detect compressor 110 surge. Microprocessor
150 controls hot gas bypass valve 134 through interface module 146.
Micro-processor 150 also keeps a record of PRV 133 position and
pressure ratio in non-volatile memory 144 for each surge event, as
described below. The conventional liquid chiller system includes
many other features which are not shown in FIG. 1. These features
have been purposely omitted to simplify the drawing for ease of
illustration.
Methods and systems consistent with this invention self calibrate
adaptively by finding the surge points as the chiller operates.
This Adaptive hot gas bypass (Adaptive HGBP or AHGBP) process
creates a surge boundary which represents the actual surge curve,
not a linear approximation. This is accomplished by electronically
detecting compressor surge when it takes place and storing in
non-volatile memory 144 numerical values which represent the
compressor head and chiller load when the surge takes place. In the
preferred embodiment, the numerical values represent the control
pressure ratio, as defined below, and PRV position for each
detected surge condition. In this way, the control panel 140
remembers where surge took place and can take the appropriate
action to prevent surge from occurring in the future by referencing
the values stored in memory.
Different parameters can be used to represent the compressor head.
For example, the method in U.S. Pat. No. 4,248,055 uses compressor
liquid temperature (CLT) to represent compressor head. According to
U.S. Pat. No. 4,282,719, which is incorporated by reference, the
pressure ratio is a better representation of compressor head than
the CLT. The pressure ratio is defined as the pressure of the
condenser minus the pressure of the evaporator, that quantity
divided by the pressure of the evaporator. While both CLT and
pressure ratio can be used in the application of the present
invention, the present preferred method is to detect and use the
pressure ratio.
According to U.S. Pat. No. 4,248,055, the difference between the
evaporator returning chilled water temperature (RCHWT) and leaving
chilled water temperature (LCHWT) can be used to represent the
chiller cooling load. While those parameters can be used with the
broadest aspect of this invention, in the preferred embodiment this
invention uses the pre-rotation vane (PRV) position to represent
chiller cooling load. Use of the PRV position minimizes variations
due to flow. Further, because the control is self-calibrating,
applications in which full load corresponds to partial open vanes
should not present a problem.
In the preferred embodiment, the method and system disclosed in
U.S. Pat. No. 5,764,062, which is incorporated by reference, is
used to detect a surge condition. When a valid surge event occurs,
the process of the invention detects and/or determines the
parameters of load and compressor head. Preferably, the process of
the invention detects and determines the current PRV position and
calculates the current pressure ratio, and then subtracts a small
margin. According to the invention, data is organized relative to a
PRV index value. For instance, a given PRV position is converted
into a percentage from zero to 100%. A current PRV index value of 1
could represent a PRV percentage of zero to 5%. A current PRV index
value of 2 could represent a PRV percentage of 5% to 10%, etc. This
method of determining the PRV index is exemplary only. Another,
preferred method is described below and in FIG. 6.
The process then accesses a table of all possible PRV index values.
Each PRV index has one control pressure ratio associated to it.
FIG. 2 shows an example of such a table and a plot of the PRV index
versus the control pressure ratio. The PRV index ranges from 1 to
20, and the stored control pressure ratios are represented by the
small letters `a` through `t`. The slope of the curve in FIG. 2 is
generally positive. The stored control pressure ratios correspond
to the sensed pressure ratios for a given PRV index value, minus a
small preselected margin. This table is stored in non-volatile
memory 144. Alternatively, the table can store other information
such as the evaporator pressure, the condenser pressure, the PRV
position, among other data that may be useful for determining the
conditions under which surge takes place.
If a surge is detected at a given PRV position and no control
pressure ratio is stored at the PRV index value corresponding to
that PRV position, the process stores the current pressure ratio,
minus a small margin, as the stored control pressure ratio at that
PRV index. The small margin is defined by the user and is
programmable through control panel keypad.
The hot gas bypass valve is opened or closed based on a comparison
of periodically sensed values of the current pressure ratios with a
stored control pressure ratio in the table, at a given PRV index.
If the current pressure ratio is greater than the stored control
pressure ratio, the HGBP valve 134 is opened by an amount
proportional (by using a proportion coefficient) to the difference
between the current pressure ratio and the stored control pressure
ratio. This corresponds to operating point A in FIG. 2. The
proportion coefficient may be programed through control panel 140.
As time progresses, if the current pressure ratio increases above
the stored control pressure ratio stored in the table, the HGBP
valve 134 is opened further to eliminate surge. The valve 134
starts to close as the current pressure ratio decreases toward the
stored control pressure ratio in the table.
If the current pressure ratio is less than or equal to the stored
value in the table, the valve 134 remains closed because this
corresponds to normal operation. This corresponds to operating
point B in FIG. 2.
If the characteristics of the system changes so that compressor 110
surges while operating at a point on or below the curve in FIG. 2,
the stored control pressure ratio in the table is decreased
incrementally. This automatically causes the HGBP valve 134 to open
more in order to stop surge. Once the surge condition has ceased
the final value stored in the table represents the new surge
boundary associated with that PRV index. Instead of decreasing the
stored control pressure ratio, it is possible to increase the
proportion coefficient, which would also automatically cause the
HGBP valve 134 to open more in order to stop a surge. Under other
circumstances, it is possible that the system characteristics can
change so that it would be beneficial to increase the stored
control pressure ratios instead of decreasing them. In this
situation, it is possible to adaptively increase the stored control
pressure ratios by control methods well known in the art.
The above process continues as chiller load conditions change and
therefore is self calibrating. In this way, the table of stored
control pressure ratios is created, revised and maintained and
reflects where the surge boundary is at a given time so that HGBP
valve 134 is opened and closed at the appropriate chiller operating
points. The table may not necessarily store a control pressure
ratio point for each PRV index because the vanes may not operate
above partially open conditions for some applications. For
instance, the PRV percentage may never reach 95 to 100% and thus
PRV index value of 20 may not have a stored control pressure ratio
associated to it. On the other hand, if a surge is detected at a
PRV index with no stored control pressure ratio, the sensed
pressure ratio is used to create a stored control pressure ratio
(by slightly decreasing the sensed ratio).
FIGS. 3A, 3B, and 3C show a flow chart of the AHGBP control process
consistent with this invention. This flow chart, and ones that
follow, contain variables and constants, which are included in
parentheses in the description below.
Microprocessor 150 executes the AHGBP control process once per
second, although it is not limited to this particular period of
time. When the AHGBP control process starts, the absolute value of
the leaving chilled water 128S temperature (LCHWT) rate of change
(lchwt.sub.13 rate) is compared to the programmable stability limit
(stability_limit) (step 1). Temperature sensor 170 measures the
LCHWT. The stability limit, if exceeded, represents a dynamic
condition that invalidates storing control pressure ratios. If the
LCHWT rate is greater than the stability limit (step 1), then the
stability timer (stability_timer) is checked (step 2). In the
preferred embodiment, the stability limit is 0.3.degree. F. per
second. If the timer has expired (step 2), then a surge hold-off
timer (surge_hold_off_timer) is started (step 3) in order to create
a window of time for storing control pressure ratios in the case
where a surge creates the unstable LCHWT condition. Control
pressure ratios are stored in a sub-process discussed below and
shown in FIGS. 4A, 4B, 4C. The surge hold-off and stability timers
are checked in that sub-process. The stability timer is reset to
its starting time (step 4) in order to assure that a time delay has
occurred after the unstable condition has subsided.
Next, the current pressure ratio (dp_p) is assigned the value of
((Condenser Pressure/Evaporator Pressure)-1), which is equal to
((condenser pressure -evaporator pressure)/evaporator pressure)
(step 5). The pressure ratio should only have positive numbers.
Therefore, if the pressure ratio is negative (step 6), it is
assigned the value of zero (step 7). Next, the average pressure
ratio (dp_pa), is assigned the average value of the past N pressure
ratios, including the current pressure ratio (step 8). In the
preferred embodiment, N is equal to ten. Averaging the pressure
ratio prevents erroneous values from fluctuations due to surges.
Then, the timers used in this process are updated (step 9).
Updating the timers involves decreasing their values until they
reach zero.
While this AHGBP process is executed, a separate surge detection
process continuously detects whether surge conditions are present
in compressor 110. As stated above, the preferred method of
detecting surge conditions is discussed in U.S. Pat. No. 5,764,062.
When the surge detection process detects a surge condition, it then
"validates" the surge condition. A "valid" or "validated" surge is
not only when surge conditions are present, but when there is a
high confidence that a surge is actually occurring. When the surge
detection process detects a valid surge, it flags it by setting a
variable (surge) to TRUE.
If surge conditions are not detected in the compressor (validated
or not) (step 10), the PRV position (prv) is stored in a memory
buffer location (prv_prior_to_surge) (step 11) to provide an
accurate indicator of the PRV position prior to surge. If surge
conditions are detected in the compressor (validated or not) (step
10), the PRV position stored in this memory buffer location remains
what it was at the beginning of the surge condition.
Next, if the surge delay timer has elapsed (step 12), the validity
of the surge condition is checked (step 14). The surge delay timer
prevents overwriting the previously stored control pressure ratios
if another surge occurs immediately after the present surge.
Therefore, the timer provides a time period that allows the system
to adjust to action taken by the by the process to the original
surge. This timer is discussed and initialized in a sub-processes
described below and in FIGS. 4A, 4B, and 4C. If a valid surge is
detected (surge=TRUE), the values of the PRV position prior to
surge (prv_prior_to_surge) and average pressure ratio (dp_pa) are
stored in temporary variable locations (plot_prv and plot_dp_p,
respectively) (step 15). If conditions permit, they are recorded,
i.e. stored in the table (step 16), which is explained in detail
below and in FIGS. 4A, 4B, and 4C. The surge condition
(surge_condition) is acknowledged (step 17) by indicating this on
the control panel user display. Then, the surge flag is cleared
(FALSE) (step 18). Finally, the Hot Gas Bypass Valve sub-process is
performed (step 19), which is described below and in FIGS. 5A, 5B,
and 5C. The HGBP Valve sub-process determines the amount of valve
opening or closing.
If the surge delay timer has not elapsed (step 12), the surge flag
is cleared (FALSE) (step 13) and the Hot Gas Bypass Valve
sub-process is performed (step 19). The surge flag is cleared step
13 and 18) because the AHGBP process took action or is currently
taking action to take the system out of any validated surge. The
surge detection process, discussed above, will set the surge flag
(surge) if necessary.
The point recording sub-process (step 16) is described in FIGS. 4A,
4B, and 4C. This process executes whenever a valid surge is
detected (step 14). This process takes the PRV position before
surge (plot_prv) and the average pressure ratio (plot_dp_p) and
stores them as control parameters into a table, such as one shown
in FIG. 2, if the appropriate qualifications are met.
First, the process checks if the system conditions are stable and
the LCHWT is operating at set-point. It does this by checking
whether the current LCHWT is within plus or minus 0.5.degree. F. of
its set-point (setpoint) and the temperature control has been
stable for 60 seconds (stability timer) or it is within 8 seconds
of the start of new unstable LCHWT condition (surge hold-off timer)
(step 20). If these conditions are met, then the current PRV index
(prv_index) is assigned a value based on the PRV position just
before the surge event (step 22). The stability timer
(stability_timer) and the surge hold-off timer
(surge_hold_off_timer) are described above and in FIGS. 2A, 2B and
2C. The set-point is a temperature programmed by the user through
the control panel 140. In the preferred embodiment, the set-point
temperature is 44.degree. F. Calculation of the PRV index is
described in more detail in FIG. 6 below.
Next, if no control pressure ratio is stored in the table at the
current PRV index (surge_pts[prv_index]) (step 23) (a zero means
that no control pressure ratio has been stored), the process
searches for a stored control pressure ratio with a higher PRV
index. (steps 25, 26, and 27). The process does not search beyond
the maximum PRV index value (MAX_PRV_INDEX). In the preferred
embodiment, the PRV index ranges from zero to a maximum of 15.
If there is a higher PRV index with a previously stored control
pressure ratio and it is less than the average pressure ratio
temporarily stored (plot_dp_p) (step 28), the process assigns the
table position at the current PRV index (prv_index) the value at
the higher PRV index minus a programmable margin (surge_margin)
(step 30). This serves as a precaution against storing a value
which is greater than any value at a higher PRV index because in
the preferred embodiment the curve should have a positive slope, as
shown in FIG. 2.
If there is no higher PRV index that has a previously stored
control pressure ratio (step 28), or it is greater than or equal to
the average pressure ratio temporarily stored (plot_dp_p) (step
28), the process assigns the control pressure ratio at the current
PRV index (prv_index) with the average pressure ratio value
temporarily stored (plot_dp_p) minus the programmable margin
(surge_margin) (step 29). This stored control pressure ratio is now
the stored control pressure ratio corresponding to that PRV index.
In the preferred embodiment, the value of the programmable margin
is between 0.1 and 0.5.
If a control pressure ratio is stored in the table (step 23), then
the process subtracts from this value the programmable margin
(surge_margin) (step 24). In this case, the process is adapting and
re-calibrating to changed system conditions, as explained above. In
all cases, the minimum value a control pressure ratio may have is
0.1. If the actual value is below 0. 1, the control pressure ratio
is assigned the value of 0.1 (steps 31, 32). An average pressure
ratio of 0.1 or less is well below what would ordinarily be
calculated and is used merely as a precaution to prevent a zero
from possibly being placed in the table (because a zero indicates
that a control pressure ratio is not entered into the table at that
PRV index). At this time, a surge response is required (step 33),
and is flagged (surge_response_required), i.e. the HGBP valve needs
to be opened to stop surge.
If the LCHWT condition is not met and the temperature conditions
are not met (step 20), then the unit conditions are not stable or
the LCHWT is not operating at set-point. In this case, a control
value should not be stored in memory, but a surge response is still
needed (independent of the surge response required flag, discussed
above). Therefore, the process adds a programmable response
increment (response_increment) to the surge response
(surge_response) (step 21). The surge response is the amount the
HGBP valve is opened in order to stop surge, and its value is
determined in the HGBP valve control sub-process explained below
and in FIGS. 5A, 5B, and 5C. In all cases, the process sets a surge
delay timer (step 34) so that no control pressure ratios are stored
in memory before the system has a chance to respond to the HGBP
valve response.
The HGBP valve control sub-process (step 19) is described in more
detail in FIGS. 5A, 5B, and 5C. This sub-process determines the
valve response comprising how much the valve should be opened or
closed. Three terms contribute to the total valve response. The
first term, the set-point response, is proportional to the current
pressure ratio minus the control pressure ratio at the current PRV
index. The second term, the surge response, is the amount the HGBP
valve is opened in response to surge. This term is exclusive of the
set-point response and always returns to zero during normal
non-surge conditions.
The third term is the minimum digital to analog converter (DAC)
response. The interface module 146 comprises the DAC, which is
necessary to control signals to the HGBP valve 134. The DAC has a
minimum value (DA_MIN) it can receive, which corresponds to the
closed HGBP valve position. Thus, the total valve response is equal
to the set-point response plus the surge response plus the minimum
DAC response.
First, the PRV index is assigned a value indicative of the current
PRV position (prv) (step 35). Assigning the PRV index is explained
in more detail below and in FIG. 6. If the PRV index contains a
previously stored control pressure ratio, and the current average
pressure ratio is greater than that value (step 36), then the
set-point response is assigned the value of a proportion
coefficient (factor) multiplied by the difference of the two values
(step 38). In other words, a response is taken that opens the HGBP
valve by an amount proportional to the difference between average
pressure ratio and the stored control pressure ratio at the current
PRV index. The proportion coefficient is programmable through
control panel 140 and preferably ranges from 10 to 100.
If either a control pressure ratio is not assigned for the current
PRV index or the average current pressure ratio is less than the
stored value at that PRV index (step 36), the process checks if a
surge response requirement is flagged (surge_response_required)
(step 37) because no set-point response will take place. If a surge
response is required (step 37), then the surge response
(surge_response) is incremented (surge_response increment) (step
39). Preferably, the surge response increment is 5% of the full
scale, but it is not limited to this.
In all cases, the surge response required flag is cleared (step 40)
because no further surge response is necessary until another valid
surge takes place. If the surge delay timer and the cycle response
timers (cycle_response_timer) are expired (step 41), the surge
response component of the HGBP valve control is slowly lowered
(step 42) by a preset amount (response_decrement) toward zero to
determine whether surge occurs again. The cycle response timer
prevents the HGBP valve from opening or closing too quickly by only
allowing valve movement in periodic intervals. This preset amount
(response_decrement) is preferably 1% of the full scale. In this
way, the HGBP valve position is optimized by only allowing the
set-point response component of the HGBP control to ultimately
contribute to the valve opening in the steady state.
The surge response should not be negative. Therefore, if the surge
response is below zero (step 43), it is set to zero (step 44). If
the current average pressure ratio is less than or equal to the
stored control pressure ratio at the PRV index value (step 45), the
process subtracts the response increment from the set-point
response (step 46) so that the HGBP valve is slowly moved to its
closed position.
The set-point response should also not be negative. Therefore, if
the set-point response is below zero (step 47), the process sets
the set-point response to zero (step 48). The cycle response timer
(cycle_response_timer) is reset (step 49) so that this portion of
the HGBP valve process is executed once every 10 seconds.
The total valve response (total_value_response) is equal to the
set-point response plus the surge response plus the minimum DAC
value (DA_MIN) (step 50). The DAC has a minimum value it can
receive (DA_MIN), which corresponds to a closed valve position. The
maximum the total valve response allowed is the full scale DAC
range value (FULL_SCALE) plus the minimum DAC value (step 51,52).
The process then opens or closes the HGBP valve (step 60) in
response to the total valve response necessary by means of
interface module 146.
FIG. 6 is a flow chart of a sub-process for determining the PRV
index (prv_index) for the stored control pressure ratios. If the
PRV value (prv_value) is less than 40% (step 53), then the index
value returned (step 58) is the PRV value divided by four (step
54). If the PRV value is not less than 40% (step 53), but is less
than 100%, then the index returned (step 58) is the PRV value
divided by ten, plus six. If the PRV value is not less than 100%
(step 55) then the index returned (step 58) is the maximum value
allowed (MAX_PRV_INDEX). In the preferred embodiment, the maximum
value allowed is 15, the PRV value ranges between zero and
100%.
The specification does not limit the invention. Instead it provides
examples and explanations to allow persons of ordinary skill to
appreciate different ways to practice this invention. The following
claims define the true scope and spirit of the invention.
* * * * *